The present invention relates to means for making electrical contact with a pair of electrical resistance, spot welding electrodes and the workpieces engaged by the electrodes. More particularly, the present invention relates to means for use in dynamically measuring the voltage drop that occurs between the electrodes and between exposed surfaces of the workpieces during spot welding of the workpieces.
A need has existed in the welding industry for a device or means that can accurately and reliably detect and convey voltage signals for use in measuring the drop in voltage that occurs between welding electrodes and between the exposed surfaces of the workpieces being welded when welding current is supplied to the workpieces by the electrodes. Such a device or means is most useful for real time monitoring of the resistance spot welding process to determine weld quality, for example.
Previously, hand-held voltage probes have been used in the measurement of true dynamic alternating current voltage drops across the weld zone in resistance spot welding of the workpieces. An example of such a hand-held unit consisted of a pair of hinged insulating blocks having therebetween leafed conductive foil strips alternating with leafed nonconductive material strips for insulating the foil strips from each other. Such a probe required a considerable amount of time to correctly align the conductive strips with the electrodes and workpieces and also required someone to hold and correctly maintain engagement of the probe with the electrodes and workpieces. During the welding operation, such a probe did not reliably, accurately and repeatedly convey the voltage signals, as the probe was inadvertently moved by the person holding it in contact with the electrodes and workpieces, and the movement of the electrodes and the workpieces during welding made it difficult to maintain electrical contact. By losing contact with the electrodes or workpieces, signals were lost during the critical welding time or unwanted noise was introduced into the desired signals.
Further, a conductive strip for contacting a workpiece surface sometimes became welded to the workpiece as the result of the expulsion of metal from the weld zone. This resulted in the probe being unusable until the conductive strip or strips were replaced.
Although not designed for use in dynamic voltage measurements during welding, an electrode mounted probe has been used in the measurement of direct current static resistance across a weld zone before the application of welding current to the workpieces by the electrodes. This probe consisted of two insulating blocks, each of which was respectively fastened to an electrode of an electrode pair. Each block had a spring-loaded voltage probe with a pointed end for contacting the exposed workpiece surface; the probe was located in parallel relation to the electrode when the block was mounted thereon. Each block also had a current clamp and a voltage clamp which attached to the respective electrode for picking up the voltage and current signals at the electrodes. Each block further had separate leads from the workpiece, voltage probe, current clamp and the voltage clamp for connection to the resistance measurement means.
Such electrode mounted probes were not suitable for use in measuring the voltage drop during actual welding because of the significant level of noise induced in the probes by magnetic fields generated in the electrodes and workpieces caused by the flow of alternating current welding current therethrough. With probes disposed parallel to the electrode, the magnetic field generated by welding current was directly coupled to the probes.
It is desirable, therefore, to provide a probe structure that avoids the above problems. Such structure should have high durability and simplicity for accurate, reliable and repeatable detection and transfer of voltage signals at the electrodes and at the exposed surfaces of the workpieces during actual welding.